The present invention relates to raster scan displays and, in particular, to a method and apparatus for pseudorandomly varying the apparent phosphor decay rate of images displayed thereon.
Phosphor persistence (i.e. the rate at which the luminescence of the phosphor on a display screen decays) presents many problems to a display system designer. Specifically, provisions must be made for refreshing the phosphor, in order to maintain a display image, as well as to permit the phosphor to decay so as to permit the displaying of new images. Stroke monitor systems, such as employed heretofore in typical radar displays, exhibit desirable decay properties, but such displays are rather expensive, require a relatively large amount of control circuitry and must be operated in a darkened environment.
When implementing a raster scan radar display, however, difficulty arises in that known techniques do not produce images of the same quality as stroke monitor displays. Furthermore, the use of conventional techniques for implementing synthetic persistence on the raster scan display (e.g. by decrementing the display image memory by sectors or quadrants) has been found to produce perceptible jumps in intensity and which in turn, has proven to be very distracting and unnatural for the observer. Such unnatural appearances result from the symmetrical nature of the decrementation of the image memory, and therefore it has been found desirable to decrement the image memory in a manner other than via known symmetrical methods.
Rather, it has been found to be preferable to pseudorandomly decrement the image memory by randomly selecting the data and by randomly varying the times at which the data is decremented and written back into the image memory during refresh operations. Such pseudorandom techniques produce images of greater brightness and enable raster scan displays that possess apparent phosphor decays equivalent to that of stroke monitor displays. Accordingly, it is a primary object of the present invention to produce raster scan displays having pseudorandom synthetic persistence. This object and others will, however, become more apparent upon reading the following description and upon referring to the related drawings.